Laser hair removal is a procedure that uses a concentrated beam of light (laser) to remove unwanted hair. ... The light energy is converted to heat, which damages the tube-shaped sacs within the skin (hair follicles) that produce hairs. This damage inhibits or delays future hair growth.
Hair grows in several phases (anagen, telogen, catagen) and a laser can only affect the currently active growing hair follicles (early anagen). Hence, several sessions are needed to damage the hair in all phases of growth and force it to revert to a vellus non-colored small hair.